The contamination of hydrocarbon streams by silicon-containing compounds results principally from the use of antifoaming agents in various stages of petroleum refining, or even in production. The antifoaming agents commonly used are the polydimethylsiloxanes, known as silicone.
Silicone is used as antifoaming agent in the process of delayed coking, preventing the entrainment of solids by reducing the formation of foam, in the process, due to the low surface tension of silicone. In delayed coking the residue from the vacuum distillation of petroleum is converted thermally to coke and to lower-boiling fractions, such as coke naphtha and light and heavy coke gas oils.
The high contents of sulphur, nitrogenated compounds and olefinic compounds present in the product streams from delayed coking make treatment necessary for upgrading of the streams as components of gasoline and diesel. Subsequent processes of hydrofining (for naphtha and gas oil) and catalytic reforming (for naphtha) are commonly used.
The polydimethylsiloxanes, however, are also converted in the coke drum (or some other stage of refining in which temperatures above 400° C. are employed), preferentially forming cyclosiloxanes, of lower boiling point, and which distil preferentially in the boiling range of naphtha. Analysis of coke naphthas shows typical contents of 1 to 10 ppm Si, possibly greater, besides contents of olefins, sulphur and nitrogenated compounds that make subsequent treatment necessary.
The problem is that these compounds containing Si have an adverse effect in the subsequent treatment units and must be absent from the final fuel. The Si compounds poison the catalytic reforming catalysts and accumulate in the catalyst beds of the hydrofining units, deactivating the catalyst and shortening the campaign time. They also impede regeneration of the contaminated hydrofining catalysts, by forming a film, of SiO2 on the metallic sites of the catalyst on oxidation of the adsorbed compounds. Hydrofining catalysts are constituted of group VIII metal (normally Co or Ni) and group VI-B metal (normally Mo or W) supported on a suitable porous solid, alumina.
U.S. Pat. No. 4,176,047 discloses a process for removing Si compounds present in coke naphtha, where the Si compound is removed in a bed of material such as alumina, activated alumina or spent desulphurization catalyst (which uses alumina as support). Temperatures above 90° C., preferably 120° C. to 150° C. are used for removing the Si compounds. The stream contaminated with silicon compounds is treated before hydrofining (HF) and catalytic reforming. No information is supplied concerning the capacity for retention of Si in these conditions (amount of Si that the bed is able to retain in the claimed operating conditions).
U.S. Pat. No. 4,269,694 and U.S. Pat. No. 4,343,693 relate to the use of bauxite (aluminium ore) for the adsorption of contaminants, including silicone, in hydrocarbon streams. Bauxite is mainly composed of hydroxides and oxides of aluminium, and at lower contents iron, silica and titania. A treatment temperature of an adsorption bed of up to 320° C., more preferably between 65° C. and 177° C., is claimed, and WHSV between 1 and 5. Preferably, after trapping of the Si compounds, the hydrocarbon stream is hydrofined. U.S. Pat. No. 4,344,841 of the same inventor discloses the use of other materials in the adsorption bed, such as montmorillonite clays, silica (amorphous), and mixtures of one or the other and with bauxite. Typical bed saturation contents of 5 wt. % are reached in the aforementioned inventions.
U.S. Pat. No. 5,118,406 deals with optimization of the beds of hydrofining process reactors for ensuring greater process stability when contaminants containing Si are present in the feed. The patent discloses that catalysts with lower activity and greater area must be positioned before catalysts that are more active, with smaller area. The use of catalysts with greater area (and lower metal content and activity), with greater adsorption capacity, followed by the catalyst that is more active, permits longer campaign times at equal reactor volume.
Catalysts supported on alumina with greater area and lower metal content are available commercially, for use before the main HF catalyst. The literature suggests that a greater catalyst area results in greater capacity for retention of Si, according to Kellberg et al. (KELLBERG, L; ZEUTHEN, P.; JAKOBSEN, H. J. Deactivation of HDT catalysts by formation of silica gels from silicone oil, characterization of spent catalysts from HDT of coker naphtha using 29Si and 13C CP/MAS NMR—Journal of Catalysis, Vol. 143, No. 1, p. 45-51, 1993). Contents of up to 7.5 wt. % of Si are reached with catalysts supported on alumina of high surface area. Moreover, the authors suggest that the trapping of the organic Si compounds occurs by reaction of surface dehydration, where a silanol is anchored to a hydroxyl exposed on the surface of the alumina, eliminating H2O. Once the catalyst is saturated with Si, it cannot be regenerated: there is formation of a film of SiO2 in regeneration, which covers the metallic sites responsible for the activity of the catalyst.
U.S. Pat. No. 6,576,121 proposes the hydrofining of feedstock contaminated with Si, additionally processing a volume of water from 0.01% to 10% relative to the feed volume. It is suggested that the presence of water increases the concentration of hydroxyls exposed on the surface of the alumina, and thus increases the capacity for retention of Si. A gain in capacity of up to 22% is obtained compared to the case without treatment with water, using a standard test. However, it is known that the use of water in catalysts supported on alumina may lead to sintering and loss of catalytic activity.
Hydrodesulphurization catalysts supported on hydrotalcite, or containing hydrotalcite in the composition, are employed for selective hydrodesulphurization of naphtha from FCC (removal of the sulphur-containing compounds with less hydrogenation of the olefins), according to U.S. Pat. No. 5,441,630. Hydrotalcite is one of the lamellar double hydroxides, also called hydrotalcite-like compounds. The lamellar double hydroxides have the general chemical formula[M1-xIIMxIII(OH)2]x+(Ax/nn−)mH2Owhere
MII is a divalent cation (Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga);
MIII trivalent (Al, Cr, Mn, Fe, Co, Ni and La);
An− represents an anion of valency n−, usually inorganic (CO32−, OH−, NO3−, SO43−, ClO4−, Cl−), heteropolyacids or even anions of organic acids. Typically, 0.2≦x≦0.33 and m is less than 0.625.
The hydrotalcites are double hydroxides of Mg and Al, and the commonest composition is[Mg6Al2(OH)16](CO3)4H2Oor[Mg0.75Al0.25(OH)2]0.125(CO3)0.5H2Owith x=0.25.
Moreover, for use as support of HDS catalyst of naphtha from FCC, in the conditions of calcination the hydrotalcite loses CO2 and H2O, resulting in mixed oxide of Mg and Al, remaining thus in the typical conditions of hydrodesulphurization (temperatures above 280° C. and absence of water and CO2).
Yang et al. (YANG, W.; KIM, Y.; LIU, P. K. T.; SAHIMI, M.; TOTSIS, T. T. A study by in situ techniques of the thermal evolution of the structure of a Mg—Al—CO3 layered, double hydroxide—Chem. Eng. Sci, vol. 57, p. 2595, 2002) provide evidence of this behaviour of dehydration and decarboxylation of hydrotalcite during calcination, resulting in the mixed oxide of magnesium and aluminium. Further details on the behaviour of the hydrotalcites and lamellar double oxides can be found in the article by Crepaldi and Valim (CREPALDI, E. L.; VALIM, J. B.—Hidróxidos duplos lamelares: síntese, estrutura, propriedades e aplicações—Química Nova, Vol. 21, No. 3, p. 300-311, 1998), incorporated here as reference.
Despite the use for selective hydrodesulphurization of naphtha from FCC, in the form of its mixed oxide, the prior literature does not cite or suggest the use of the catalyst as adsorbent of Si compounds.
The importance of processes of conversion of petroleum bottoms product (heavy hydrocarbons) to light distillates, such as delayed coking, and the need for clean fuels with lower content of contaminant by means of hydrofining, as well as the continuous development presented in the prior art show that more effective processes and catalysts for removing Si are desirable, which is achieved in the present invention.
The present invention provides the use of lamellar double hydroxides, such as hydrotalcite, as support of hydrofining catalyst or adsorbent, resulting in improvement to the state of the art for retention of silicon-containing compounds that contaminate hydrocarbon streams.